1. Field of the Invention
The present invention relates to a color image transmitting method in a color facsimile machine for transmitting color image data by utilizing a public line or the like.
2. Description of the Related Arts
Hitherto, there has been a method of transmitting a color original document through a digital public line or ISDN arranged in such a manner that the data about the overall color original document is processed as color image data and the data, the whole document, is thereby transmitted as it is. However, when a full set of image data of a A4 (219.46 mm.times.297 mm) sized original document is transmitted through a line having 64 Kbps in line speed in the form of, for example, an RBG color image, it takes 739.9 seconds (12 minutes and 19.09 seconds) in a case of a binary data image of a line density of 400 dpi. What is even worse, in a case of a multivalued data image, it takes 5919.3 seconds (98 minutes and 39.03 seconds).
Therefore, a variety of color image data compression methods have been disclosed in which a color image is compressed by a variety of methods so as to be transmitted. As examples of color image compression methods of the type described above, there have been a vector quantizing method in which the image is replaced by sets of predetermined patterns and an ADCT (Adaptive Discrete Cosine Transform) which is an orthogonally conversion encoding method.
However, the above-described method in which the overall portion of the original document is transmitted as it is encounters a problem in terms of practical use since it takes an excessively long time except for a case in which accurate image data is desired to be received by the receipt side so as to have it stored in a storage.
On the other hand, the image transmission method in which an image compression is performed exhibits an advantage in that the quantity of data can be significantly reduced and the time necessary to transmit the image can be shortened if the image is compressed in accordance with a proper algorithm. However, partial loss of image information cannot be prevented by a properly selected compressing technology when the compression ratio is raised. Furthermore, another problem arises in that image data about an image which considerably changes, a smooth image, an image containing human skin and an image including characters cannot efficiently and satisfactorily be compressed.